Genetics and Disease — MCQs

Genetics and Disease Indian Medical PG Practice Questions and MCQs

Question 41: Which chromosomes are associated with autosomal dominant polycystic kidney disease I and II, respectively?

A. Chromosomes 16 and 5
B. Chromosomes 16 and 4 (Correct Answer)
C. Chromosomes 11 and 5
D. Chromosomes 11 and 4

Explanation: **Explanation:** Autosomal Dominant Polycystic Kidney Disease (ADPKD) is the most common hereditary kidney disease [1]. It is genetically heterogeneous, caused by mutations in two primary genes: 1. **ADPKD Type 1:** Caused by a mutation in the **PKD1 gene** located on **Chromosome 16p13.3**. This gene encodes **Polycystin-1**, a protein involved in cell-cell and cell-matrix interactions. It accounts for approximately 85% of cases and typically presents with a more rapid progression to End-Stage Renal Disease (ESRD) (mean age ~54 years) [1]. 2. **ADPKD Type 2:** Caused by a mutation in the **PKD2 gene** located on **Chromosome 4q21-23**. This gene encodes **Polycystin-2**, which functions as a calcium-permeable cation channel. It accounts for about 15% of cases and generally follows a milder clinical course with later onset of ESRD (mean age ~74 years) [1]. **Analysis of Incorrect Options:** * **Options A & C:** Chromosome 5 is associated with Familial Adenomatous Polyposis (APC gene), not ADPKD. * **Options C & D:** Chromosome 11 is associated with conditions like Wilms tumor (WT1) and Sickle Cell Anemia (Beta-globin chain), but not ADPKD. **High-Yield Clinical Pearls for NEET-PG:** * **Extra-renal manifestations:** The most common is **Liver Cysts**. The most serious/life-threatening is **Berry Aneurysms** (Circle of Willis), which can lead to Subarachnoid Hemorrhage. * **Diagnosis:** Ultrasonography is the primary screening tool; criteria are based on the number of cysts relative to age. * **Treatment:** **Tolvaptan** (a V2-receptor antagonist) is used to slow the increase in total kidney volume and decline in GFR.

Question 42: An 18-year-old patient presents with sebaceous cysts on the scalp and back of the neck, and an osteoma on the right mandible. Radiographs reveal multiple impacted supernumerary teeth in both jaws. These findings suggest which of the following conditions?

A. Cleidocranial dysostosis
B. Ectodermal dysplasia
C. Gardner's syndrome (Correct Answer)
D. Osteogenesis imperfecta

Explanation: **Explanation** The clinical triad of **sebaceous (epidermoid) cysts**, **osteomas** (particularly of the mandible and skull), and **supernumerary teeth** is pathognomonic for **Gardner’s Syndrome**. **1. Why Gardner’s Syndrome is Correct:** Gardner’s syndrome is a clinical variant of **Familial Adenomatous Polyposis (FAP)**, caused by a mutation in the **APC gene** on chromosome 5q21. It is characterized by the systemic manifestation of: * **Colonic polyps:** 100% risk of progression to colorectal carcinoma if not treated. * **Extra-colonic bone findings:** Osteomas (most common in the mandible). * **Dental abnormalities:** Impacted supernumerary teeth, odontomas, and congenitally missing teeth. * **Soft tissue tumors:** Epidermoid cysts, fibromas, and desmoid tumors. **2. Why the Other Options are Incorrect:** * **Cleidocranial dysostosis:** While it features supernumerary teeth, it is characterized by clavicular hypoplasia (allowing shoulders to touch) and delayed closure of fontanelles, not osteomas or sebaceous cysts. * **Ectodermal dysplasia:** This typically presents with **hypodontia** (missing teeth) and conical-shaped teeth, along with sparse hair and anhidrosis, which contradicts the "supernumerary teeth" finding. * **Osteogenesis imperfecta:** Characterized by blue sclera, hearing loss, and multiple fractures due to Type I collagen defects. It does not present with osteomas or sebaceous cysts. **High-Yield Clinical Pearls for NEET-PG:** * **CHRPE:** Congenital Hypertrophy of Retinal Pigment Epithelium is often the earliest detectable sign of Gardner’s syndrome. * **Desmoid Tumors:** These are aggressive fibroblastic tumors that often occur post-surgery in Gardner’s patients. * **Screening:** Patients with these extra-intestinal markers must undergo immediate **colonoscopy**, as malignant transformation of polyps occurs by age 40.

Question 43: Two siblings have osteogenesis imperfecta, but their parents are unaffected. Which of the following genetic mechanisms best explains this inheritance pattern?

A. Anticipation
B. Genomic imprinting
C. Germline mosaicism (Correct Answer)
D. New mutation

Explanation: ### Explanation **Correct Answer: C. Germline mosaicism** **Mechanism:** Germline (or gonadal) mosaicism occurs when a mutation happens in a precursor germ cell during early embryonic development of a parent [1]. As a result, a proportion of the parent's gametes (sperm or eggs) carry the mutation, while their somatic cells do not. This explains why the **parents are phenotypically normal** (unaffected), yet can have **multiple children** with the same autosomal dominant condition like Osteogenesis Imperfecta (OI) [1]. **Why other options are incorrect:** * **A. Anticipation:** Refers to the increasing severity or earlier onset of a disease in successive generations, typically seen in trinucleotide repeat disorders (e.g., Huntington’s disease, Fragile X). * **B. Genomic imprinting:** Involves differential gene expression depending on whether the allele is inherited from the mother or father (e.g., Prader-Willi and Angelman syndromes) [2]. * **D. New mutation (De novo):** While OI often results from new mutations, a single de novo mutation would typically affect only **one** sibling. The occurrence of the disease in **two** siblings with unaffected parents strongly points toward mosaicism in the parental germline rather than two separate, coincidental new mutations. **NEET-PG High-Yield Pearls:** * **Classic Examples:** Germline mosaicism is a high-yield explanation for **Osteogenesis Imperfecta** and **Duchenne Muscular Dystrophy** when multiple siblings are affected but parents are normal. * **Recurrence Risk:** In cases of germline mosaicism, the recurrence risk for future siblings is higher than the general population (usually 1-5%), despite the parents testing negative for the mutation in blood samples. * **OI Type 2:** This is the most severe (perinatal lethal) form and is frequently associated with this inheritance pattern in board exams.

Question 44: In which state does a dominant trait express?

A. Homozygous state
B. Heterozygous state (Correct Answer)
C. Both homozygous and heterozygous states
D. Males only

Explanation: **Explanation:** In genetics, a **dominant trait** is defined by its ability to manifest phenotypically even when only one copy of the mutant allele is present. According to Mendelian principles, a dominant allele masks the expression of a recessive allele. Therefore, the trait is expressed in the **heterozygous state (Aa)**. **Why Option B is the most accurate contextually:** While a dominant trait *can* be seen in a homozygous state, the fundamental definition of "dominance" in medical genetics is the expression of the phenotype in the heterozygote [1]. In clinical practice, most Autosomal Dominant (AD) disorders (e.g., Marfan Syndrome, Achondroplasia) are encountered in the heterozygous state because the homozygous state (AA) is often lethal in utero or results in a much more severe, distinct phenotype. **Analysis of Incorrect Options:** * **Option A (Homozygous state):** While the trait is expressed here, this state is not required for a dominant trait to manifest. This is the requirement for *recessive* traits. * **Option C (Both):** While technically true in basic biology, in medical entrance exams, the "heterozygous state" is the classic defining feature used to distinguish dominance from recessiveness. * **Option D (Males only):** This describes X-linked recessive traits in hemizygous males (e.g., Hemophilia), not the general rule for dominant traits. **High-Yield Clinical Pearls for NEET-PG:** 1. **Vertical Transmission:** AD disorders typically show vertical inheritance (seen in every generation). 2. **Achondroplasia:** A classic AD trait. Homozygosity for achondroplasia is usually lethal. 3. **Variable Expressivity:** Individuals with the same AD genotype may show different degrees of clinical severity. 4. **Reduced Penetrance:** An individual inherits the dominant gene but does not express the phenotype at all.

Question 45: Which type of Ehlers-Danlos syndrome is characterized by arterial or uterine rupture?

A. Classical EDS (Type I)
B. Hypermobile EDS (Type II)
C. Classical EDS (Type III)
D. Vascular EDS (Type IV) (Correct Answer)

Explanation: **Vascular Ehlers-Danlos Syndrome (vEDS)**, formerly known as Type IV, is the most severe form of the disorder [1]. It is caused by an autosomal dominant mutation in the **COL3A1 gene**, which leads to defective synthesis of **Type III collagen** [1]. Since Type III collagen is a major structural component of the walls of blood vessels and hollow organs, its deficiency results in extreme fragility. This leads to the hallmark life-threatening complications: spontaneous **arterial rupture** (often involving the abdominal aorta or mesenteric arteries), **uterine rupture** during pregnancy, and **hollow viscus perforation** (commonly the sigmoid colon). [1] **Analysis of Incorrect Options:** * **Classical EDS (Type I/II):** Caused by mutations in **COL5A1 or COL5A2** (Type V collagen). It is characterized by skin hyperextensibility, "cigarette paper" (atrophic) scarring, and joint hypermobility, but not typically spontaneous organ rupture. [1] * **Hypermobile EDS (Type III):** This is the most common subtype. While the genetic basis is often unknown, it presents primarily with generalized joint hypermobility and chronic pain; it lacks the severe vascular risks of Type IV. [1] **NEET-PG High-Yield Pearls:** * **Gene Association:** vEDS = *COL3A1*; Classical = *COL5A1/A2*; Kyphoscoliotic = *Lysyl hydroxylase* deficiency. [1] * **Clinical Sign:** Patients with vEDS often have a characteristic facial appearance (thin nose, prominent eyes, lobeless ears) and "aged" appearing hands (acrogeria). * **Diagnosis:** Skin biopsy for collagen typing or genetic testing is definitive. Avoid invasive imaging (like conventional angiography) due to the high risk of vascular injury.

Question 46: Which of the following findings is NOT typically seen in Marfan syndrome?

A. Scoliosis
B. Hyperlaxity
C. Pes planus
D. Hypospadias (Correct Answer)

Explanation: Explanation: Marfan syndrome is an autosomal dominant connective tissue disorder caused by a mutation in the **FBN1 gene** on chromosome 15, which encodes **fibrillin-1**. This protein is a major component of extracellular matrix microfibrils. The clinical manifestations primarily involve the skeletal, ocular, and cardiovascular systems [1]. **Why Hypospadias is the Correct Answer:** Hypospadias is a congenital malformation of the male urethra where the opening is on the ventral surface of the penis. It is typically related to hormonal imbalances or localized developmental failures during embryogenesis. It is **not** a feature of connective tissue dysplasia and is not associated with Marfan syndrome. **Analysis of Incorrect Options:** * **Scoliosis:** Skeletal overgrowth is a hallmark of Marfan syndrome. Asymmetric growth of the ribs and vertebrae frequently leads to spinal deformities like scoliosis or kyphosis. * **Hyperlaxity:** Deficiency in fibrillin-1 leads to weakened connective tissue in joint capsules and ligaments, resulting in generalized joint hypermobility (measured by the Beighton score) [1]. * **Pes planus:** Also known as flat feet, this occurs due to laxity of the ligaments supporting the medial longitudinal arch of the foot. It is a common minor skeletal criterion in the Ghent nosology. **High-Yield Clinical Pearls for NEET-PG:** * **Cardiovascular:** The most common cause of death is **Aortic Root Dilatation** leading to dissection or rupture. Mitral Valve Prolapse (MVP) is also common. * **Ocular:** The classic finding is **Ectopia Lentis** (dislocation of the lens), typically occurring **upward and outward (Superotemporal)** [1]. * **Skeletal Signs:** Look for **Steinberg sign** (thumb sign) and **Walker-Murdoch sign** (wrist sign). * **Differential:** Homocystinuria presents similarly but features downward lens dislocation, intellectual disability, and a prothrombotic state.

Question 47: What is the genetic defect in Klinefelter's syndrome?

A. XYY
B. XXY (Correct Answer)
C. XXX
D. XO

Explanation: **Explanation:** **Klinefelter Syndrome (47, XXY)** is the most common sex chromosome disorder in males, occurring in approximately 1 in 600 live births. The correct answer is **XXY** because the syndrome is characterized by the presence of at least one extra X chromosome in a male phenotype [1]. This usually occurs due to **maternal meiotic non-disjunction** (failure of chromosomes to separate during egg formation). **Analysis of Incorrect Options:** * **XYY (47, XYY Syndrome):** Also known as Jacob’s Syndrome. These individuals are phenotypic males, often tall, with normal fertility but an increased risk of learning disabilities. * **XXX (47, XXX Syndrome):** Known as Triple X Syndrome. These are phenotypic females who are often asymptomatic or may present with tall stature and mild developmental delays. * **XO (45, X):** This is **Turner Syndrome**. It is characterized by a female phenotype with short stature, webbed neck, and streak ovaries (primary amenorrhea) [3]. **Clinical Pearls for NEET-PG:** * **Classic Triad:** Small, firm testes (testicular dysgenesis), gynecomastia, and infertility (azoospermia) [2]. * **Hormonal Profile:** Low Testosterone, **High FSH, and High LH** (Hypergonadotropic Hypogonadism) due to loss of feedback inhibition [2]. * **Physical Findings:** Tall stature with "eunuchoid" body proportions (long legs, narrow shoulders, wide hips) and sparse body hair [2]. * **Associated Risks:** Increased risk of **Male Breast Cancer**, Germ cell tumors (extragonadal), and Mitral Valve Prolapse [4]. * **Histology:** Fibrosis and hyalinization of seminiferous tubules with **Leydig cell hyperplasia** [2].

Question 48: The Philadelphia chromosome is associated with which genetic event?

A. Deletion
B. Break DNA
C. Balanced translocation (Correct Answer)
D. Balanced transcription

Explanation: ### Explanation **1. Why "Balanced Translocation" is Correct:** The Philadelphia chromosome (Ph) is the hallmark of **Chronic Myeloid Leukemia (CML)** [1]. It results from a **reciprocal (balanced) translocation** between chromosomes **9 and 22**, denoted as **t(9;22)(q34;q11)** [1]. * In this process, the *ABL1* proto-oncogene from chromosome 9 moves to the *BCR* (Breakpoint Cluster Region) on chromosome 22 [1]. * This creates a **BCR-ABL1 fusion gene**, which encodes a constitutively active **tyrosine kinase** [1]. This protein drives uncontrolled cellular proliferation and inhibits apoptosis. It is "balanced" because genetic material is exchanged without significant loss or gain of DNA [1]. **2. Why Other Options are Incorrect:** * **Deletion (A):** While deletions occur in many cancers (e.g., 5q- syndrome), the Philadelphia chromosome specifically requires the fusion of two distinct gene segments from different chromosomes. * **Break DNA (B):** DNA breaking is a prerequisite step for any translocation, but it is a generic process, not the defining genetic event of the Philadelphia chromosome [2]. * **Balanced Transcription (D):** This is a distractor term. Transcription is the process of mRNA synthesis; the Philadelphia chromosome is a structural **cytogenetic** abnormality at the DNA level. **3. Clinical Pearls for NEET-PG:** * **Disease Associations:** Found in >95% of CML cases [1]. Also seen in **B-ALL** (indicates poor prognosis) and occasionally in AML. * **Targeted Therapy:** The discovery of this translocation led to the development of **Imatinib (Gleevec)**, a tyrosine kinase inhibitor (TKI) that revolutionized CML treatment [1]. * **Diagnosis:** Identified via **Karyotyping** (shows the shortened chromosome 22), **FISH** (detects the fusion gene), or **RT-PCR** (most sensitive for monitoring minimal residual disease).

Question 49: What type of inheritance is seen in MELAS syndrome?

A. Mitochondrial (Correct Answer)
B. Autosomal Dominant
C. Autosomal Recessive
D. X-linked

Explanation: **Explanation:** **MELAS syndrome** (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes) is a classic example of **Mitochondrial Inheritance** (maternal inheritance) [2]. It is most commonly caused by a point mutation in the mitochondrial DNA (mtDNA) at position A3243G in the *MT-TL1* gene, which encodes tRNA leucine. 1. **Why Mitochondrial is correct:** Unlike nuclear DNA, mitochondrial DNA is inherited exclusively from the mother (as the sperm contributes negligible cytoplasm to the zygote). A hallmark of this inheritance is **Heteroplasmy**, where a cell contains a mixture of both mutant and normal mtDNA. The clinical severity depends on the proportion of mutant DNA in specific tissues [1]. 2. **Why other options are incorrect:** * **Autosomal Dominant/Recessive:** These involve mutations in the nuclear genome (chromosomes 1–22). While some mitochondrial proteins are encoded by nuclear DNA, the specific MELAS phenotype is linked to the mitochondrial genome [1]. * **X-linked:** This involves genes on the X chromosome. MELAS affects both males and females equally, but only females can transmit the disease to their offspring. **Clinical Pearls for NEET-PG:** * **Classic Triad:** Stroke-like episodes (before age 40), Encephalopathy (seizures/dementia), and Lactic Acidosis [2]. * **Muscle Biopsy:** Shows **"Ragged Red Fibers"** (Gomori trichrome stain) due to subsarcolemmal accumulation of abnormal mitochondria. * **Other Mitochondrial Diseases:** MERRF (Myoclonic Epilepsy with Ragged Red Fibers), LHON (Leber’s Hereditary Optic Neuropathy), and Kearns-Sayre Syndrome [2]. * **High-Yield Fact:** All children of an affected mother are at risk, but children of an affected father will not inherit the disease.

Question 50: Which of the following statements is not true regarding Fragile X syndrome?

A. Large testis
B. Pigmented nevi (Correct Answer)
C. Large ears
D. Prominent forehead

Explanation: Fragile X syndrome is the most common cause of **inherited intellectual disability** and the most common monogenic cause of **autism**. It is caused by an expansion of the **CGG trinucleotide repeat** in the *FMR1* gene on the X chromosome, leading to transcriptional silencing and a deficiency of the FMRP protein [1]. **Why "Pigmented Nevi" is the correct answer:** Pigmented nevi (moles) are not a characteristic feature of Fragile X syndrome. While skin findings like hyperextensible skin may occur due to connective tissue dysplasia, pigmented lesions are more classically associated with conditions like **Neurofibromatosis Type 1** (Café-au-lait spots). **Analysis of Incorrect Options:** * **Large Testis (Macro-orchidism):** This is a hallmark feature of Fragile X, typically appearing post-puberty. It is caused by an increase in interstitial fluid and connective tissue. * **Large Ears & Prominent Forehead:** These are classic craniofacial features. Patients typically present with a long, narrow face, large protruding ears, a prominent forehead, and a prominent jaw (prognathism). These features result from underlying connective tissue abnormalities. **High-Yield Clinical Pearls for NEET-PG:** * **Genetics:** X-linked dominant inheritance with **anticipation** (symptoms become more severe in successive generations). * **Repeat Sequence:** **CGG** (Mnemonic: **C**GG = **C**hin, **G**iant **G**onads) [1]. * **Diagnosis:** PCR or Southern blot analysis (to detect the number of repeats). Cytogenetic analysis shows a "fragile site" when cultured in folate-deficient medium. * **Associated Features:** Mitral valve prolapse, joint hypermobility, and behavioral issues (ADHD, hand-flapping, social anxiety).

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Principles of Medical Genetics

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Genetic Testing and Counseling

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Single Gene Disorders

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Chromosomal Disorders

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Mitochondrial Diseases

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Pharmacogenomics

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Cancer Genetics

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Genetics of Common Diseases

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Epigenetics and Disease

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Genetic Basis of Developmental Disorders

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Ethical Issues in Medical Genetics

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Gene Therapy and Precision Medicine

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